Much outdoor construction activity occurs at night, particularly when it relates to highway improvements, since traffic counts are greatly reduced at night and inconvenience to the general public is minimized. A common way of illuminating a worksite is with a trailerable portable light tower. These light towers generally are assembled on small towable trailers which include an engine driven generator set operably connected to a light bar assembly which is mounted at the top of a mast which can be raised and lowered as needed. Oftentimes, the mast can be raised to an elevation of 20 to 30 feet above the trailer.
The mast must be capable of being raised and lowered. A portable trailer having a light tower mast that extends 30 feet into the air cannot be easily towed around without lowering the mast. The masts are generally not made of unitary construction since that would entail lowering the mast by pivoting it down where it would extend 30 feet back out behind the trailer. Instead, they are normally made of four or five telescoping pieces which can be lowered into a convenient stationary mast base so that the top of the trailer when the mast is fully lowered is no more than about 8 feet above ground. Such a trailer is then easily portable and can be moved about conveniently, and can even be stored or repaired indoors.
There are a number of ways of raising and lowering a mast, which include hydraulic systems, which are expensive and require additional equipment, namely a hydraulic pump and hydraulic plumbing and the supply of hydraulic fluid so as to extend the mast up and later retract it. The most common method of extending and retracting the mast is a wire cable and winch assembly.
A problem, which occurs more frequently than is desired, is that the cables used to extend and retract the mast will occasionally snap or part. When this occurs, all of the telescoping pieces come crashing down, one into the other, and if the operator happens to be standing underneath the mast at the time it comes crashing down, the operator can be seriously injured or even killed as the mast itself will typically weight 200 to 300 pounds. When they crash down, that light bar can land with a force in excess of 700 pounds which can be fatal to an unfortunate operator who happens to be standing underneath the mast at the time it comes crashing down.
Various attempts at providing some kind of safety catch for elevated light tower masts have been attempted in the past. In the case where a hydraulic system is used to raise and lower the mast, any failure of the hydraulic system, for example a burst hose, will simply result in the mast slowly dropping, and there is adequate time for anybody under the light tower mast to clear the area before the mast and its associated light tower is fully down. In the case of cable-powered elevation systems, one common practice has been to use some sort of a safety clevis pin to lock one or more pieces of the mast in place. However, this has not been a satisfactory arrangement because clevis pins are commonly misplaced and/or lost, or because the operator fails to install the clevis pin properly, or because the mast is raised to some intermediate position where the holes through which the clevis pin is to be inserted do not properly line up.
What is needed is an automatic latching system which by default is always operable to catch the mast and hold it in position in the event that the hoisting cables fail.